The present invention relates to a method of joining aluminum to titanium by welding and by using so much welding energy that the titanium and the aluminum melt at the interface at the joint. In addition, the present invention relates to weldment in which titanium has been joined to aluminum by welding.
In the electrolysis of sodium chloride, a titanium mantle is nowadays often used, at least on that side of the cell to which the anodes are attached. In this case the basic material of the anodes is usually titanium. The cell mantle to one side of which the anodes are attached, or the ends of the anodes passing through the cell mantle, is/are then attached with a bolt joint to aluminum current conductors. Such bolt joints cause transfer resistances and thereby losses of energy, and they are therefore undesirable.
Because it is exactly in electrolytic cells that there has been a need for joining aluminum and titanium to each other, several methods have been developed for this purpose.
In British Pat. No. 1,125,493 rolling, explosive welding and resistance welding are mentioned as methods. Since in rolling a great force is required for producing a metallurgical bond between aluminum and titanium, the method is cumbersome. It must be limited to very thin sheets, and the method is not suitablefor attaching thick conductor rails or the aluminum support structures of the tanks. Furthermore, rolling has to be carried out in machine shops equipped with special devices, and it is not suitable for being carried out on site. Titanium and aluminum can be joined to each other by explosive welding. However, the method is very cumbersome and expensive for attaching thick current rails to the titanium mantle of an electrolytic cell. In addition, explosive welding can be carried out only in plants provided with special equipment. If the titanium mantle of an electrolytic cell is in this manner `coated` with an aluminum sheet, the aluminum current rails must further be attached by separate welding to the aluminum current conductors. In addition, making pipe units in such a Ti-Al mantle is complicated, owing to these two materials of the mantle, if the situation is compared with a mantle consisting of only titanium. Since aluminum is a good conductor of electricity and heat, only thin aluminum sheets can be attached to titanium sheets by resistance welding. Thus, the maximum thickness indicated in the said patent for the Al sheets to be attached is approx. 3 mm.
British Pat. No. 1,127,484 mentions soldering and brazing in addition to explosive welding, in which case a flat titanium sheet is attached to relatively thick aluminum base sheets. Soldering and brazing are very cumbersome operations, because first an oxide film has to be removed from both the titanium and the aluminum, whereafter the pieces must be kept free of oxide, for example in a vacuum or in a shield gas, until the welding has been carried out. Furthermore, several fillers are usually required.
The patent also includes a mention of the casting, from aluminum, protrusions which protrude from the cell. The casting operation is difficult to carry out, especially when what is in question is relatively thick current rails. If casting were used for attaching the current rails, an oxide film left on the surface of the titanium, the melting point of the oxide film being much higher than the casting temperature, would produce a layer having electric resistance and thereby losses of energy when conducting current. Also for this reason, casting cannot be used for current rails. The method is also out of the question in the attaching of the support structures of large tanks, because the method would be difficult to carry out in practice and the said oxide film would weaken the strength of the structure.
In British Pat. No. 1,522,622, fusion welding methods comparable to casting are used. In this case, aluminum pins are attached by friction welding to the mantle of a titanium electrolytic cell. The pins are rotated by a hydraulic motor so that the contact surfaces of the pins, against the titanium, melt. The method is very cumbersome, because when conductor rails are used, the pins must also be connected to the rails in some manner. In addition, this joining method is limited to rotatable pieces.
The above-mentioned patent also mentions capacitor discharge welding. This is also a kind of resistance welding, in which the pieces to be welded are pressed against each other, whereafter an electric discharge is conducted through the pieces, at which time the aluminum melts. The high electrical and thermal conductivity of aluminum sets limits on the thickness of the pieces to be joined or on their cross sectional surface, thus limiting the method to use with relatively thin pieces or pieces with a small cross sectional surface, because in thick pieces electricity and heat are transferred rapidly in the lateral direction, in which case sufficient thermal energy for carrying out the welding is not obtained in the areas to be welded.
The special brazing method disclosed in German patent application DOS No. 2735059 requires several steps and fillers as well as special equipment. For this reason the method is cumbersome.
In British Pat. No. 1,237,090 it is disclosed that titanium could be welded by TIG welding to aluminum. It must be noted that what is in question is not actual gas arc welding but a fusion process, since no filler material (welding wire) is used. It is noted in the Patent that, in order to be resilient, the parts to be joined must be sufficiently thin. It can be noted that it is not possible to join thicker pieces together by the method. The method cannot be used for normal welded joints, since the composition of the melt is uncontrolled and, for this reason, pieces of Ti and Al cannot be reliably joined to each other. On the other hand, the patent does not disclose how the melt could be controlled except that the thickness of the Al must be at minimum 4 times greater than the thickness of the titanium to be joined to it. Such thickness requirements would greatly limit the range of constructions.
All such complicated methods are, of course, due to the fact that it has not been possible to apply gas arc welding to welding titanium to aluminum, because brittle compounds of the metals are thereby produced.
In U.S. Pat. No. 4,264,426, however, gas arc welding has been applied to attaching aluminum current conductors directly to, for example, the titanium mantle of an electrolytic cell.
In Finnish Pat. No. 58,164, gas arc welding has been used for the attaching of aluminum support structures and stiffeners to titanium tanks, whereby an uncomplicated and strong support structure has been obtained for the tank, and the previously used complicated constructions have been avoided.
Even though it has been possible to apply gas arc welding, the controllability of the method has not, however, been the best possible.